International Journal of Scientific & Engineering Research Volume 4, Issue 2, February-2013ISSN 2229-55181Field Measurement of Vertical Strain in AsphaltConcreteMd Rashadul Islam1, and Rafiqul A. Tarefder2Abstract— Recently developed Mechanistic-Empirical Pavement Design Guide (MEPDG) determines the probable total rutting by summing upthe deformations of all layers of the trial pavement. If all distresses outcomes including rutting are within the specification the trial section is designed for construction. Therefore, vertical deformation (or strain) needs to be measured to validate the MEPDG for local conditions. However,no direct procedure for measuring the vertical strain in flexible pavement is available to this date. Traditionally, Earth Pressure Cell (EPC) is usedto measure the vertical stresses at different layers of pavement. Then, vertical strain is calculated using the measured stress and the stiffness ofthe corresponding material. The present study describes a procedure to measure the vertical strain of asphalt concrete based on field instrumentation. The vertical strain is measured using the Vertical Asphalt Strain Gauge (VASG) in an instrumented pavement section on Interstate 40 (I40) in the state of New Mexico, USA and compares the results with numerical model developed using ABAQUS. The numerical model is validated with stress responses measured with installed four EPCs. The stiffness input of the numerical model is obtained from Falling Weight Deflectometer (FWD) test. Promising agreement is observed between the strains measured in the field and determined from the numerical model.Therefore, the VASG can be considered an effective sensor to measure the vertical strain of asphalt concrete in flexible pavement.Index Terms— Asphalt concrete, Vertical stain, Vertical stress, Vertical asphalt strain gauge, Earth pressure cell, Falling weightdeflectometer test, Numerical analysis, Validation—————————— ——————————1INTRODUCTIONRutting is one of the major distress outcomes of Mechanistic-Empirical Pavement Design Guide (MEPDG). It is determined by summing up of permanent deformations of alllayers along the wheel path [1]. Therefore, measuring verticalstrain is so important for accurate rut prediction. Eq. 1 is usedto calculate the permanent deformation in asphalt concrete: () ()ℎ () 10(1)where () Accumulated plastic vertical deformation inHot Mix Asphalt (HMA) (in.),() Accumulated plastic vertical strain in HMA layer,() Elastic strain calculated by structural response modelat mid-depth of each sub layer,ℎ Thickness of HMA layer, (in.),n Number of axle-load repetitions,T Pavement temperature (⁰F), Depth confining factor, function of HMA total depth andthe depth concerned,, , Local field calibration constants,, , Global field calibration constants,Therefore, () is the only parameter needed to be measured to determine the plastic deformation. Accurate measurement of vertical strain is necessary for probable rut estimation.Measuring vertical deformation or strain of asphalt 1Ph.D.Student, Dept. of Civil Engineering, University of New Mexico, MSC01 1070, 1 University of New Mexico, Albuquerque, NM 87106; USA. PH(505) 363-6902; email: [email protected] is much cumbrous. Several researchers measured thevertical deformation or vertical stress, not the vertical strain.Scullion et al. [2] used Multi Depth Deflectometer (MDD) tomeasure the vertical deflection under Falling Weight Deflectometer (FWD) and vehicle loads. A 38 mm diameter (1.5")and 2.1 m (7.0 ft.) deep borehole was prepared in the pavement to install the sensors. The researchers measured the relative and the total permanent deformation. However, determining actual deformation from this data is near impossible.Schaevitz HCD-500 Linear Variable Displacement Transducer (LVDT) was used to measure the vertical deformationsof the asphalt and concrete and subgrade base in MinnesotaRoad Research Project [3]. A PVC pipe of 2.4 m (8 ft.) long and112 mm (4.5") diameter was inserted into the pavement inwhich a reference rod of 3.6 m (12 ft.) long and 25 mm (1.0 in.)diameter was inserted into the pipe. The pipe was mounted to25 mm (1.0") below the pavement surfaces. The LVDT wasplaced at a certain depth on the reference cap. The process isreally cumbersome and the inserted 112 mm pipe also deteriorates the quality of pavement and the hole may act as stressfocal point. This installation procedure was not followed inany future studies.National Center for Asphalt Technology (NCAT) performedseveral instrumentation projects in 2003 and 2006 and studiedthe vehicle-pavement interaction and effects on pavementdeterioration [4], [5]. Vertical stress and horizontal strain ofasphalt concrete were measured in these studies with Geokon3500 Earth Pressure Cells (EPCs) and Construction TechnologyLaboratories Inc. (CTL) supplied Horizontal Asphalt StrainGauges (HASGs). Twelve flexible pavements were instrumented in Virginia Smart Road project [6], [7]. The researchersinstalled Dynatest Strain Gauges to measure horizontal strainin asphalt concrete and Geokon 3900 EPC to measure verticalProfessor, Dept. of Civil Engineering, University of New Mexico.MSC 01 1070, 1 University of New Mexico, Albuquerque, NM 87106; USA. IJSER 2013

International Journal of Scientific & Engineering Research Volume 4, Issue 2, February-2013ISSN 2229-5518stress at the layer interfaces.The deformation of the asphalt concrete can be calculatedby using the vertical stress with the stiffness of the asphaltconcrete. However, HMA stiffness is fully dependent on ageof the pavement, temperature profile of the pavement andfrequency of loading. These factors make the output quitechallenging to be accurate.A new procedure is described in the present study tomeasure the vertical strain in asphalt concrete. CTL has developed Vertical Asphalt Strain Gauges (VASGs) which are capable of measuring vertical strain in asphalt concrete [8]. Six sensors are installed and the responses are evaluated with thenumerical analysis. A promising performance is observed between the measured vertical strain and the numerical modeldetermined vertical strain. The details of this sensor, workingprinciple, installation procedure and functionality are described in the following sections.2OBJECTIVESThe main objective of this study is to evaluate the performanceof the VASG to measure the vertical strain of asphalt concrete.Numerical model is developed to determine the strain response at the VASG sensors’ elevation. This model is validatedwith the stresses responses measured with the pressure cellsinstalled at four other depths.3VERTICAL ASPHALT STRAIN GAUGE3.1 GeneralThe VASG measures the vertical strain in asphalt concrete forrepeated traffic loading. It has a sharp leg to be inserted intothe underlying base layer. The gauge area is secured by twostainless steel circular plates as shown in Fig. 1.2plates and hand compacted to acquire the required density. Ithas Teflon polymer coated braided lead wire to avoid electrical noise and withstands temperature up to 400 ⁰F (205 ⁰C).Fig. 1 also shows that the cable is inserted into aluminum conduit to protect it from larger aggregates.3.2 Principle of OperationThe resistance to electrical current provided by a conductor ofuniform cross-section is given by the well-known preliminaryphysics equation (Eq. 2) given below: (2)where R is the resistance of the conductor in ohms, is theresistivity of the material, and L and A are the length andcross-sectional area of the conductor respectively. If the conductor is stretched the length increases and the cross-sectionalarea decreases. The relationship of the change in resistancewith change in length and diameter is expressed as in Eq. 3: (3)The term FG is commonly referred to as gauge factor, orstrain sensitivity factor and is a function of the alloy characteristics, L0 and R0 are the initial length and resistance of the conductor respectively and is the change in resistance forchange in length ( ). The change of resistance produces somevoltage of a bridge circuitry (Fig. 2). An analog to digital converter changes the analog voltage to a digital signal that isinterpreted by data acquisition system.Fig. 2. Wheatstone bridge. Ra, Rb, Rc arethe fixed resistances. R x is changeable.Vin is zero for balanced condition. It yields the followingcondition as in Eq. 4: Fig. 1. Vertical Asphalt Strain Gauge. The cable is insertedinto aluminum conduit to protect it from larger aggregates.The bottom plate helps the sensor to remain in positionwithout tilting. The top plate transfers the applied load uniformly on the sensors. HMA is inserted between these two(4)where Ra, Rb, Rc and Rx are the four resistances of the bridge.The bridge becomes unbalanced if any resistance changes. Oneof the resistors, in this case Rx, is replaced by strain gauge tomeasure the strain. If the gauge experiences a change in resistance, the voltage across the meter is then related to thechange in resistance. The strain in the gauge can be calculatedfrom this voltage if gauge factor is known. The manufactureprovides this factor and is to be verified in laboratory.IJSER 2013

International Journal of Scientific & Engineering Research Volume 4, Issue 2, February-2013ISSN 2229-55183.3 Installation ProcedureA small hole of around 0.375 mm diameter and 100 mm deepis drilled. Some sand binder mixtures are applied to fill thehole and cover the surroundings. The gauge is gently presseddown into the hole until the bottom plate comes into full contact with the surface. The top plate of the gauge is removedand HMA mixture is placed around each gauge immediatelybefore paving. The mixture is then compacted with tampingrod without damaging the sensor. The top cap is then replacedand some mix is applied on top of it. Then, contractors canpave over it.43PG 70-22 is 4% (by weight of mixture). The maximum aggregate size is 25 mm (1 in.). About 5% of the materials passesthrough No. 200 sieve (0.075 mm).EARTH PRESSURE CELLEarth pressure cell is used to measure the vertical stresses inbase, subbase and subgrade. Two circular stainless steel platesare welded together along their boundaries as shown in Fig. 3.Fluid is used to fill the narrow gap between the plates. Thefluid gets stressed when vertical load is applied on top of it. Apressure transducer is connected with stainless steel tube. Thetransducer transforms this fluid pressure into an electrical signal (usually voltage) which is noticed by the data acquisitionsystem.Fig. 4. Longitudinal profile of the instrumentation section. Thelayers of the section and the elevations of the sensors arepresented.A total of twenty two vertical and horizontal strain gauges,four pressure cells, three moisture probes, and six temperatureprobes were installed inside the pavement at different layers.Sensor data are gathered continuously by a high speed dataacquisition system and recorded in a computer.EPCs were installed at the layer interfaces and at the middle of the base. A 300 mm (12.0") square, was prepared to accommodate the 225 mm (9.0") diameter EPC as shown in Fig.5. The depth of the hole was approximately 60 mm. A 75 mm(3.0") wide by 75 mm (3.0") deep trench was prepared withpick axes to accommodate the cable.Fig. 3. Earth pressure cell. The two circular steel platesare welded along their peripheries.5FIELD INSTALLATIONS AND DATA COLLECTIONPavement strain data was collected from an instrumentedpavement section on Interstate 40 (I-40) east bound lane atmile post 141 near Albuquerque in the state of New Mexico,USA. The cross section of the instrumented section is shown inFig. 4. The section has four layers with 300 mm (12") Surface,144 mm (5.75") Base, 200 mm (8.0") Process Place and Compact(PPC) and Subgrade.The asphalt concrete used in this pavement is a densegraded SuperPave (SP) mix, type SP-III, which is widely usedin the New Mexico Department of Transportation (NMDOT).This mix containes plant screened Recycled Asphalt Pavement(RAP) materials around 35%. Performance Grade (PG) binderFig. 5. Installation of EPC on top of the base. Some fine materials were placed at the top and at the bottom of the plate.The bottom 25 mm (1.0") was filled with sieved PPC material passing #8 and compacted with Marshall Hammer tomake a smooth and level bed. Then, the sieved PPC materialpassing #16 was placed approximately 18.75 mm (0.75") thick.The EPC was then placed such that it attained the exact eleva-IJSER 2013

International Journal of Scientific & Engineering Research Volume 4, Issue 2, February-2013ISSN 2229-5518tion in the section. The EPC was also leveled as best as possible as shown in Fig. 5. All positions and elevations of the sensors were surveyed and recorded for analysis. Six verticalstrain gauges were installed at the bottom of the asphalt layer.The holes for the gauges were made by drilling machine. Somesand-binder mix was placed into the hole and then the gaugeswere inserted into it.of each at 0.6 m (2 ft.) interval for a length of 18 m. The resulting surface deflections at different radial distances weremeasured with the sensors. The tests data were analyzed bybackcalculated software ELMOD. The stiffnesses were calculated using the linear elastic theory and the method of equivalent thickness. The average stiffness of both these methodswas used numerical analysis. The pavement temperature during the FWD was not measured. However, both the FWD testing and sensors’ data recording were conducted at the sametime to avoid the temperature incongruity. The average stiffnesses are measured to be 2410 MPa (350 ksi), 185 MPa (27ksi), 158 MPa (23 ksi) and 103 MPa (15 ksi) for HMA, base,PPC, and Subgrade respectively.7Fig. 6. Installation of VASG. The top steel plate is removed for applying some sieved mixtures over it.Prior to paving over the gauges, the top plate was taken offas shown in Fig. 6 and some -#4 HMA mixtures were appliedand compacted with tamping rod. The top plate was then returned and covered with more sieved mixtures. Then, thetruck dumped the mixture over the sensors and compactedwith no vibration. The density of the compacted mix waschecked through field coring and measuring with nucleardensity measuring device.64FINITE ELEMENT MODEL (FEM)An axis-symmetric model is developed in ABAQUS to determine the vertical strain at the installed VASGs’ elevation. Themodeled section has four layers with 300 mm Surface, 144 mmBase, 200 mm Process Place and Compact (PPC) and Subgradeas shown in Fig. 8. The radius of the section and the tire pressure are used as 2.5 m (100") and 150 mm (6.0") respectively.The moduli of the material are taken from the Falling WeightDeflectmeter (FWD) test. These values are 2410 MPa, 185 MPa,158 MPa and 103 MPa for HMA, base, PPC and subgrade respectively. Poisson’s ratios are used as 0.35, 0.4, 0.45 and 0.45for HMA, base, PPC and subgrade respectively [9].FALLING WEIGHT DEFLECTOMETER TEST (FWD)FWD tests were conducted on the final lift of the pavement inlate May, 2012 in early morning as shown in Fig. 7.Fig. 8. The developed FEM model. The section has four layerswith 300 mm Surface, 144 mm Base, 200 mm PPC and Subgrade.Fig. 7. Conducting FWD on top of the HMA. Three typesof loads were applied and the resulting surface deflections were measured.Two types of loads were used, 9 and 12 kips with two dropsA linear elastic analysis is performed with axisymmetric,second order, and quadrilateral element, CAX8. No lateraldisplacement at the edges of the pavement, no vertical displacement at the foundation and no slip between layers areassigned as boundary conditions. The tire inflation pressure ofeighteen-wheel vehicle 0.83 MPa (120 psi) is assigned in staticmanner. The vertical stress and the vertical strain contourplots are presented in Fig. 9 and in Fig. 10 respectively. Thevertical stress decreases with the depth. However, the verticalstrain pattern depends on the stiffness of the material andIJSER 2013

International Journal of Scientific & Engineering Research Volume 4, Issue 2, February-2013ISSN 2229-5518sharply changes with change in material. The detailed resultsare discussed in following section.5Typical vertical stress response and vertical strain strain response are presented in Fig. 11 and Fig. 12 respectively. Theseare the screen shots of the outputs. These show that the responses of all the wheels are not same. In this study, the reading of the third axle wheel is considered. The calibration factors are provided by the manufacturers and verified in thelaboratory.TABLE 1VERTICAL STRESS COMPARISON TO VALIDATE FEMDepthTop of BaseAt Mid-BaseTop of PPCTop of SubgradeFig. 9. Vertical stress distribution in the section. The area underthe load is highly compressed and decreases with depth. Faraway, the stress is almost zero.Fig. 11. Typical stress response for an eighteen-wheel vehicle. The x-axis shows the time (sec) of collection and y-axisshows the output volatage. The voltage is multiplied by thecalibration factor to get the stress response in psi.Fig. 10. Vertical strain distribution over the section. The highest compressive strain occurs at the top of the base. Sometensile strain develops beside the tire load.8Vertical Stress 790.03350.0337RESULTS AND DISCUSSIONThe VASG measures the vertical strain at 268 mm (10.7")depth. The EPC measures the stress response at 300 mm (12"),369 mm (14.75"), 444 mm (17.75") and 750 mm (30") depths.Same depth response is needed to evaluate the results. FEM isdeveloped to determine the vertical strain at 268 mm depth.The model is validated with the measured stresses at differentdepths. The stresses are measured with the installed EPCs.Therefore, the FEM response at 268 mm is representative ofthe EPC response. The stress responses at different locations ofthe pavement are compared with numerical analysis. The results are shown in Table 1. The vertical stress on the top ofbase is 0.0957 MPa which is very close to the measured value(0.0961 MPa). This value decreases to 0.0337 MPa on the top ofsubgrade and the measured value is 0.0333 MPa. The percentage error between stresses (FEM and measured) varies -0.43 to1.04%, which is negligible. Thus, the EPCs responses validatethe numerical model. Now, FEM determined strain is suitablefor comparison with field measured strain.Fig. 12. Typical vertical strain for an eighteen-wheel vehicleat the depth of 268 mm. The x-axis shows the time (sec) ofcollection and y-axis shows the output volatage. The voltageis multiplied by the calibration factor to determine the strainresponse in µε.The strain value at the VASG location is determined by thenumerical analysis. The FEM output is 196 µε. The responsesfor VASGs are taken for 21 passes of the vehicle. The dataIJSER 2013

International Journal of Scientific & Engineering Research Volume 4, Issue 2, February-2013ISSN 2229-5518ranges 192-203 µε with the mean (same with median) value of198 µε and standard deviation of 1.63% of the mean. The distribution of the data is plotted in Fig. 13. It shows that themean and the mdian value are almost same. The maximumand the minimum value are 203 and 192 µε respectively. Thismean value of the measured strain is 1.02% higher than theFEM (or EPC) response. This difference is very negligible.Therefore, it can be concluded that VASG correctly measursthe vertical strain in asphalt concrete.ACKNOWLEDGMENTThe authors highly acknowledge the research funding fromNew Mexico Department of Transportation. The authorswould like to express their sincere gratitude and appreciationto Virgil Valdez, Parveez Anwar, Jeff Mann and Robert McCoyfor their regular support, advice and suggestions.REFERENCES[1][2][3][4][5]Fig. 13. Boxplot of the measured strain data. The minimumand the maximum values are 192 and 203 µε respectively,with the mean and the median value of 198 µε and standarddeviation of 1.63% of the mean value. The lower whisker islarger than the upper whisker.[6][7]9CONCLUSIONSThe measured stress responses with the EPC at different locations of the pavement are correlated with numerical study.This correlation validates the developed numerical model. Thenumerical response (vertical strain at 268 mm depth) of thismodel is then compared with the VASG response. Resultsshow an outstanding match between the FEM and the VASGreadings. Therefore, the VASG can be considered an effectivesensor to measure the vertical strain of asphalt concrete.6[8][9]AASHTO, “Mechanistic-Empirical Pavement Design Guide.” A manualof Practice, AASHTO. July 2008, Interim Edition, American Association of State Highway and Transportation Officials, Washington D.C, 2008.T. Scullion , J. Uzan, J. Yazdani, and P. Chan “Field Evaluation of theMulti-Depth Deflectometer.” FWHA Report No. TX-88/1123-2, 1988,submitted by Texas Transportation Institute, Texas A&M University,Texas.H. B. Baker and Bath, M. R. (1994). ''Minnesota Road Research Project:Load Response Instrumentation Installation and Testing procedures.''Report No. MN/PR-94/01, Submitted to Minnesota DOT, St. Paul,Minnesota 55155.D. Timm, A. Priest, and T. McEwen, ''Design and Instrumentation ofthe Structural Pavement Experiment at the NCAT Test Track.'' NCATReport 04-01, 2004, Submitted to Alabama DOT, Auburn, Alabama.D. Timm ''Design, Construction and Instrumentation of the 2006 TestTrack Structural Study.'' NCAT Report 09-01, 2009, Submitted toAlabama DOT, Auburn, Alabama.A. Loulizi, I. Al-Qadi, S. Lahour, and T. Freeman, ''Data Collection andManagement of the Instrumentated Smart Road Flexible PavementSections.'' Transportation Research Record 1769, Paper No. 01-2668,2001, Washington D. C.W. Nassar, ''Utilization of Instrument Response of SuperPave Mixes at theVirginia Smart Road to Calibrate Laboratory Developed FatigueEquations.'' PhD Dissertation, Department of Civil andEnvironmental Engineering, Virginia Polytechnic Institute and StateUniversity, Blacksburg, Virginia, 2001.www.ctlgroup.comM. R. Islam, M. U. Ahmed and R. A. Tarefder R. A. “PerformanceEvaluation of the Embedded Sensors in I-40 Pavement in New Mexico.” 2ndInternational Conference for Sustainable Design, Engineering andConstruction, ASCE, Fort Wroth, Texas, 2012, pp. 519-526.IJSER 2013

The vertical strain is measured using the Vertical Asphalt Strain Gauge (VASG) in an instrumented pavement section on Interstate 40 (I- . installed Dynatest Strain Gauges to measure horizontal strain in asphalt concrete and